Improvement in the fuel consumption of automobiles is a very important subject for saving energy. This subject is important for human society from the standpoint of decreasing generation of carbon dioxide (CO2) as one of the means for preventing the global warming.
With respect to the lubricating oil for internal combustion engines (the engine oil), it is known that the decrease in the viscosity is effective as the means for decreasing the fuel consumption since friction loss (resistance of stirring) in the engine oil can be decreased. Therefore, it is necessary for improving the fuel consumption of an automobile that viscosity of the base oil constituting the major portion of the engine oil be decreased.
However, when the viscosity of the engine oil is decreased, problems arise in that wear resistance which is essential for the engine oil decreases and that the consumption of the oil increases due to the increase in vaporization of the oil and the increase in the amount of the oil discharged to the outside accompanied with the blow-by gas of the engine. It is the actual situation that decreasing the viscosity of the engine oil (the base oil) is difficult.
To overcome the problem of the decrease in the wear resistance due to the decrease in the viscosity of the base oil among the above problems, the addition of an agent for improving the load carrying property such as an oiliness agent, an antiwear agent and an extreme pressure agent can be considered. It is proposed that an organomolybdenum compound is added as the friction modifier (for example, refer to Patent References 1 and 2).
To overcome the problem of the increase in the consumption of the oil, the use of a synthetic oil having a low viscosity and a high viscosity index is attempted. However, the synthetic oil is expensive, and the sufficient effect is not always obtained by the use of the synthetic oil having a high viscosity index. When a mineral oil-based base oil having an insufficient viscosity index is used, no effective means for overcoming the problems have been found. Therefore, an engine oil of the mileage-saving-type which can achieve the sufficient decrease in the consumption of the oil even when a widely used base oil having a low viscosity is used, has been desired.
In general, the viscosity of the engine oil is decreased when the engine is working since the engine oil is heated at a high temperature. Therefore, there is the possibility that the lubricating property such as the wear resistance of the engine oil becomes insufficient during the driving when a base oil having a low viscosity is used.
To overcome the above problem, a so-called multi-grade engine oil which comprises a macromolecular compound as the viscosity index improver and suppresses the decrease in the viscosity at high temperatures is used.
However, the multi-grade engine oil described above has a problem in that the viscosity index improver, which is a macromolecular compound, is subjected to a shear stress since the engine oil is subjected to a shear stress in the engine, and suppressing the decrease in the viscosity at high temperatures becomes less effective during the use of the engine oil. Therefore, it is required for the engine oil, in particular, for the engine oil of the energy saving type, that the viscosity at low temperatures be low so that the friction loss at low temperatures is small, that the viscosity at high temperatures be high (which means the decrease in the viscosity be small), and that the shear stability be excellent. These properties are, in general, evaluated based on the CCS viscosity at a low temperature such as −35° C. (JIS K 2010), the kinematic viscosity at 100° C. (JIS K 2283) and the high temperature high shear viscosity (ASTM D 4741), respectively.    [Patent Reference 1] Japanese Patent Application Laid-Open No. Heisei 6(1994)-313183    [Patent Reference 2] Japanese Patent Application Laid-Open No. Heisei 5(1993)-163497